In electromechanical systems there is a frequent need for a constant frequency electrical output from a variable speed mechanical input, or conversely, a need for a constant speed mechanical output from a variable frequency electrical input. Many methods for satisfying this need have been advanced. The present invention provides a solution using a homopolar (acyclic, Faraday) direct current (DC) machine, the field flux of which is varied continuously to produce an alternating current (AC) output. The frequency of the AC output is governed by the frequency of the field excitation and is independent of the shaft speed of the mechanical input.
A conventional separately-excited DC generator produces an AC electrical output if the separate field is excited with an AC current. However, if AC field excitation is used with a conventional DC generator with multi-turn armature and associated commutator, extensive sparking is present at the commutator. Significant power losses and excessive commutator wear are present and the resulting operation is not satisfactory. A homopolar (acyclic, Faraday) DC generator operates without commutation, so such a generator can readily be separately excited with AC current. Since the homopolar generator has only one armature turn, the AC electrical output is characterized by small voltage and large current.
The homopolar, or unipolar, DC machine was invented by Faraday in 1831. Regardless of its geometric form, the unipolar DC machine is characterized by a single turn rotor, so commutation is not required. Since it is a 1-turn DC machine, the rotor output voltage is small and the output current large.
There have been many inventions intended to circumvent the basic high-current problem of the unipolar DC machine. None of these inventions has proved commercially attractive, however, so the machine currently is used only for special low-voltage applications.
In U.S. Pat. No. 1,516,254 which issued on Nov. 18, 1924 to George S. Towar, a unipolar machine is disclosed which utilizes variable field excitation to produce AC output. The low-voltage AC output is connected to a transformer to obtain a high-voltage output from the transformer secondary. However, since the transformer primary current is so large, the connection between the unipolar machine and the transformer primary is of critical importance. To minimize electrical losses, the transformer primary connection must be made as short as feasible.